Adding Objects into action editor
You can apply the same action with two different armatures. If they contain no bones with the same name, then the action will not collide. An action contains animation graphs with names. If the action is assigned, the graphs are associated with the correspondingly named bones.

If you assign it to the ball and chain, existing keyframes won't be removed. Once you keyframed the ball and chain bones, there will be ball and chain curves created. If the character doesn't have any bone named ball or chain, these curves won't interfere with it.